The present invention relates to reduced energy consumption remote sensing from a satellite. The invention is applicable to improving the energy requirements of satellites particularly in the case of a scatterometer, that is to say apparatus for sensing radar back-scatter, which can be used to sense remotely the speed and direction of wind at the surface of the sea and, by way of example, the invention will be described with reference to such an application.
A wind measuring apparatus of this kind utilizes a radar system and is based on measurement of the microwave back-scatter coefficient of the surface of the water at sea. In fact, it has been found that gravity waves, that is the swell of the sea, are very little dependent on the wind, whereas capillary waves that is to say very small waves which "ripple" the surface of the sea, depend closely on the wind. In the accompanying drawings, FIG. 1 is a diagrammatic view of a gravity wave 1 which is "modulated" by capillary waves 2. The wave length of the gravity waves is much bigger than that of radar transmissions, while capillary waves, on the contrary, have wave-lengths comparable with radar and there is an interaction between the capillary waves and the radar signal. For angles of incidence .theta. greater than 25.degree., the Bragg condition is fulfilled and the radar echo of the ocean is essentially a function of the frequency spectrum of the ocean. The back-scatter coefficient .sigma..degree. is a function of the speed and the direction of the wind.
It is therefore possible to determine the speed and the direction of the wind at the surface of the ocean from a satellite. Accordingly, scatterometers have been mounted on satellite platforms in order to measure the wind velocity. Such apparatus is described in the publication "IEEE Journal of Oceanic Engineering" Volume OE-S No. 2 (April 1980) pages 138 to 154. They comprise a "fan" antenna having a beam which is wide in elevation and narrow in azimuth. The receiver includes a doppler filter which defines measurement cells corresponding to quadrangular scans of the ocean strip illuminated by the radar beam. For reasons of efficiency, the angle of incidence is limited to 55.degree., which corresponds to the most remote measurement cell from the satellite.
FIG. 2 of the accompanying drawings shows a satellite 5, measuring wind characteristics for angles of incidence .theta. with the ground ranging from 25.degree. (closest cell) to 55.degree. (furthest cell). Thus, with eight measurement cells, the speed and direction of the wind can be measured over a width of 400 Km, each cell being a square of side 50 Km. This is illustrated in FIG. 3 where a satellite 10 is shown having two antennas, one forwards 11, and one rearwards 12, whose beam outline on the measurement strip of ocean 13 is 400 Km wide (8.times.50 Km). The broken line 14 indicates the satellite ground track and the beam of each antenna makes an angle of 45.degree. with the direction of movement of the satellite.
The prior art scatterometers for wind measurement on board satellites have a certain number of disadvantages.
The transmission requirements are calculated for the worst case, that is to say to obtain a satisfactory measurement for the most remote cell. However, since the backscatter coefficient is much higher for the smaller angles of incidence which correspond to the closest cells, the return signal is about 100 times larger than that needed to satisfy the measurement accuracy requirements. For close cells there is an overabundance of energy which leads to energy wastage. This is particularly troublesome for an onboard apparatus where all means of reducing consumption are required to increase the life-time of the flight.